The present invention relates to power supply systems used to operate lower voltage accessory motor drives on an electric vehicle (including battery, hybrid, and fuel cell vehicles) using power drawn from a high voltage bus.
Electric vehicles are most widely known for their use of electric motors to provide drive power to the vehicle wheels. However, these vehicles commonly use various accessory drive motors to operate a variety of different vehicle systems. Some examples of these accessory drive motors include liquid coolant pumps, traction control motors, climate control compressor motors, electronic power steering pumps, as well as blowers and other fans. The accessory motors can be either brush-type or brushless DC electric motors and are available at the 350-400 Vdc ratings needed to operate from the high voltage buses typically used on electric vehicles. However, this high voltage operation raises a number of concerns including arcing and other commutation problems with use of the conventional brush-type DC motors. Accordingly, lower voltage motors are sometimes used that can be driven from an intermediate voltage power bus operating at, for example, a fixed 42 Vdc. This lower voltage supply can be produced using a DC-DC converter that draws operating power from the high voltage bus and develops the fixed 42 v supply using conventional DC-DC conversion techniques. Both brush-type and brushless DC motors can be connected to this intermediate bus for safer, more reliable operation.
Many of the vehicle systems operated by these accessory drive motors require a variable speed output of the motor. For brush-type DC motors operating off a fixed intermediate voltage, speed control is typically accomplished using an external speed control module that is connected between the intermediate bus and motor. The speed control module receives a speed control signal from the appropriate controller for the system being operated by the motor, and this control signal is used by the speed control module to vary the DC voltage provided to the motor. For brushless DC motors operating off the fixed intermediate voltage, the speed control signal is provided to a set of speed control electronics incorporated into the motor itself. This arrangement, while permitting operation of the different types of DC motors from a lower voltage supply, nonetheless has a number of inherent disadvantages. First, operation of the different motors from a fixed supply with separate speed control modules and/or electronics is unnecessarily inefficient in terms of power usage, especially at lower loads and speeds. This is due in part to the common practice of using two, cascaded pulse width modulated (PWM) stagesxe2x80x94one in the DC-DC converter and the other in the speed control circuitry. Secondly, the use of an external speed control module for the brush-type motors adds to the overall system cost and requires additional space within the vehicle. Similarly, the use of a brushless DC motor with this system requires a more expensive motor having the speed control electronics built in.
Another known system design that provides increased operating efficiency and perhaps somewhat lower cost involves the use of different DC-DC converters for the two different types of DC motors. In this system, the brushless motors are operated in the same manner as described above, using a fixed intermediate supply with speed control being provided by the motor itself. However, the brush-type motors are operated each with a dedicated DC voltage supply having a variable output that is determined in accordance with the speed control signal. This eliminates the need for the external speed control module and the concomitant second PWM stage. However, while this system design may improve cost and efficiency somewhat, it still suffers from some of the same disadvantages as the first system topology described above; namely, it requires different supplies for the different types of motors, uses two cascaded PWM stages for the brushless motors, and utilizes a more expensive brushless motor having integral speed control electronics.
It is therefore a general object of this invention to provide an accessory motor drive power supply system having a simplified, low cost design that provides improved power efficiency over conventional designs. It is also preferably an object of this invention to provide such a power supply system that permits speed control of both brush-type and brushless motors using a uniform speed control interface for the various vehicle systems involved.
In accordance with the present invention there is provided an electric vehicle power supply system that utilizes a single power supply design to provide efficient, variable speed motor control to both brush-type and brushless DC motors. The power supply system includes at least one brush-type and brushless DC motor along with a first DC-DC converter that provides operating power to the brushless DC motor and a second DC-DC converter that provides operating power to the brush-type DC motor. Both converters have a power input connected to the electric vehicle""s high voltage bus and each includes a data input for receiving a speed control signal indicative of desired motor speed, as well as an output for providing a motor drive signal to its associated DC motor. Each of the converters is operable in response to its received speed control signal to convert operating power from the high voltage bus into a lower voltage variable motor drive signal that is provided to its associated motor. The variable drive signal can be either a variable DC voltage signal or a PWM signal.
With this arrangement, a single converter design, including a single speed control interface, can be utilized to operate both brush-type and brushless DC motors. Furthermore, the system permits more efficient operation of the brushless DC motors since it obviates the need for speed control electronics on the motor itself and, in doing so, eliminates the relatively inefficient use of cascaded PWM stages.